I'd even add one more element to their list: phone calls are more distracting than a conversation because of the pauses. When two people are having a conversation, since one or the other is constantly talking, it becomes sort of a constant background noise (which is easy for me to block out, say, while reading on a train). But overhearing just one half of a phone call, an irregular sequence of noise-pause-noise-pause, is impossible for me to filter out because each time the person starts talking again is a new distraction.

Anyway, for all these reasons, cell phones in a quiet space like a library are always going to be a problem - even in a not-necessarily-quiet library like mine. Until we can get a good white noise generator* that would drowned it out, perhaps what we really need are dedicated spaces for people to have cell phone conversations.